How to perform switching power supply design for medical equipment

　　With the rapid development of medical electronic technology, the types of medical equipment are increasing. Medical equipment is increasingly closely related to modern medical diagnosis and treatment. Any medical equipment cannot do without a safe and stable power supply, and most of them are switching power supplies. In the process of daily diagnosis and treatment, it is often encountered that the equipment cannot be used due to power failure. At this time, the clinical medical engineers of medical service institutions need to combine their own experience and professional knowledge to provide rapid and efficient services to the clinical department. Due to the particularity of medical equipment, the power supply of the equipment is poorly interchangeable, and some even lack technical drawings, which brings great inconvenience to maintenance work.

　　Medical equipment switching power supplies can generally be divided into two categories: AC/DC and DC/DC. The input of the primary power supply AC/DC converter is 50/60Hz, 220V AC, which must be rectified and filtered. Large filter electrolytic capacitors are indispensable, and the AC input must be added with EMC filtering and safety standard devices. The secondary power supply DC/DC converter is used for power conversion. It is the core part of the switching power supply. In addition, there are startup, overcurrent and overvoltage protection, noise filtering and other circuits. The output sampling circuit detects the output voltage change and compares it with the reference voltage. The error voltage is amplified and pulse width modulation (PWM) circuit, and then the duty cycle of the power device is controlled by the drive circuit, so as to achieve the purpose of adjusting the output voltage. The basic structure is shown in Figure 1.

　　Figure 1 Basic structure of medical equipment switch

　　Switching power supply damage can be roughly divided into the following categories based on the damaged components: ① damage to inductive, capacitive and resistive devices; ② damage to power semiconductor devices; ③ damage to PWM IC; ④ damage to optocoupler; ⑤ damage to others, such as crystal oscillator, fan and other power supply devices.

　　According to the power supply workflow, it can be divided into: ① AC input failure; ② DC/DC converter failure; ③ drive circuit failure; ④ PWM circuit failure; ⑤ sampling circuit failure. There are many types of switching power supply failures, which cannot be listed in detail here. The following is a discussion of typical maintenance techniques in the above two categories combined with actual maintenance examples.

　　1 Input circuit failure

　　The input circuit of the switching power supply of medical equipment generally includes switches, fuses, AC anti-interference circuits and soft-start circuits. It is easy to find faults in switches, fuses and AC anti-interference circuits. If the switch is damaged, it can be replaced directly, but if the fuse is damaged, it is best to check whether the load is seriously short-circuited, and replace it with a fuse of the same ampere to monitor the total input current when the power is turned on. Failure of the AC anti-interference circuit is generally due to the failure of the capacitor due to long-term use. The soft-start circuit is one of the protection circuits of the switching power supply. The input circuit of the switching power supply is mostly designed with a rectifier plus capacitor filter circuit. At the moment of closing the input circuit, since the starting voltage on the capacitor is 0, a large instantaneous impact current will be formed.

　　For this reason, medical equipment switching power supplies generally have a soft start circuit in the input circuit to prevent inrush current.

　　Common soft start circuits include thermistor anti-inrush current circuit, SCR-R circuit, circuit composed of relay and resistor, circuit using timing trigger and current limiting resistor, and circuit composed of optocoupler thyristor and bidirectional thyristor triggered by zero crossing. The following is a brief explanation of its working principle using thermistor anti-inrush current circuit as an example: Thermistors are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC). The normal resistance of PTC is relatively low. When there is an excessive abnormal current flowing through, the resistance value of PTC increases rapidly due to its own heating, and the resistance becomes larger, which plays a role in current limiting; NTC thermistor has a larger resistance value at the moment of power supply connection, which can achieve the effect of limiting inrush current.

　　When the circuit is in normal working condition, the resistor generates heat and its resistance value decreases.

　　NTC thermistor anti-inrush current circuit Due to the thermal inertia of the thermistor, it takes time to restore the original resistance value. When the power is turned off and then turned on quickly, it cannot play a current limiting role.

　　Many infusion pumps and some low-power medical equipment power supplies use PTC thermistor current limiting or NTC thermistor surge current protection circuit design. Among them, PTC thermistors are easily damaged when encountering lightning or strong currents, and they are always in a low-resistance state and will burn out when powered on. NTC thermistors often have open circuit failures, resulting in a primary power supply DC without AC access.

　　2 Photocoupler failure

　　Optical Coupler is also called photoelectric coupler, or optocoupler for short. It is a device that uses light as a medium to transmit electrical signals. Usually, infrared light-emitting diodes and photosensitive semiconductors are packaged in the same tube shell. When an electrical signal is added to the input end, the light-emitting diode emits light, and the photosensitive semiconductor receives the light to generate an electrical signal, which flows out from the output end, thereby realizing the "electric-light-electric" conversion. It is widely used in signal isolation, switching circuits, pulse amplification, solid-state relays (SSR) and other circuits. In addition, linear optical couplers can be used to form an optical coupler feedback circuit, and the duty cycle can be changed by adjusting the current at the control end to achieve the purpose of precise voltage regulation.

　　Optocouplers can achieve electrical isolation and have the advantages of strong anti-interference ability, long service life, and high transmission efficiency [3]. However, circuit failures caused by degradation of optocoupler performance are still common in switching power supplies for medical equipment.

　　Example 1: Many clinical medical engineers have come into contact with the power supply of the Philips BV25 X-ray machine. Among them, the failure to start the machine due to poor performance of the photocoupler has almost become a common problem of this power supply. The BV25 main power supply adopts a contactless soft start circuit design. When 220V is connected, a transformer provides a group of 28V and multiple groups of 7V power supplies. After rectification and voltage regulation, the 28V is rectified and stabilized to obtain a +15V voltage to provide power to the power control board, and 7V is supplied to each group of photocouplers. If H1 on the power board is green, it can be roughly judged that the 28V and 7V outputs are normal. Poor performance of thyristors V1-V3 and photocouplers (4N25) B1-B6 will cause startup failure. To determine whether V3 is damaged, it needs to be removed and measured, otherwise it is easy to misjudge.

　　Example 2: OHMEDA 2000 infant incubator, the temperature continues to rise after reaching the set value, reporting "E013". The maintenance manual shows "Header not switching off". After eliminating the thermal switch failure, the most likely cause is the poor performance of the optocoupler in the SSR. After replacing the device, the incubator works normally.

　　Medical equipment switching power supplies are like other switching power supplies. Power devices are indispensable. Among them, power diodes, thyristors (SCRs) and power field effect tubes are more commonly used. During the maintenance process, power devices are the focus of inspection. Damage to such devices will cause power-on protection or fuse burnout. When such devices are found to be damaged during maintenance, in addition to replacing devices with the same parameters, the peripheral high-voltage capacitors and current limiting or current detection resistors must also be checked.

　　Example 1: When the Alcon Universal II ultrasonic emulsification device is turned on, the panel has no display, the "Standby" light flashes, and the switching power supply makes a "squeaking" sound, which roughly indicates that the power supply has protection action. The power supply uses PWM ICs such as UC3842, UC3843 and UC3854. The current detection terminals of each IC indicate overcurrent, and the voltage of each power supply terminal jumps. After excluding damage to the PWM IC and peripheral circuits, the power devices were checked in detail. The switch tube (IRF460) of one power supply was broken down. After replacing the field effect tube, the peripheral circuit was checked again. It was found that the C26 high-voltage capacitor (1KV) connected to it had broken down. After replacing C26, the power was turned on, and the main +24V output was normal. All the connections of the machine were restored, and the voltage of each group was normal and the whole machine worked stably.

　　Example 2: The monitor of SHIMADZU OPESCOPE 50N X-ray machine has no display and the indicator light flickers. The total power supply of the X-ray machine is 220V, while the monitor power supply is 110V. Before sending it for repair, the operator added 220V to the monitor alone, but the indicator light did not light up. The monitor power supply adopts STR 54041 switching power supply thick film module design, and its DS pole has been broken down, and D1722 has been broken down. After replacement, the dummy load is connected and the power output of each line is normal. After the circuit connection is restored, the indicator light is on, and there is a "clicking" sound in the machine, but there is still no display. After inspection, it is found that the line tube Q9 and the fuse resistor R71 are damaged. After replacement, the whole machine works normally.

　　4 PWM IC and peripheral circuit failure

　　The combination of power control chip and switch tube is widely used in medical equipment switching power supply, and one power supply may even be used in multiple places. The basic working principle of PWM switching voltage or current stabilization power supply is that when the input voltage, internal parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal, adjusts the conduction pulse width of the main circuit switch device, and makes the switching power supply and the output voltage or current and other controlled signals stable.

　　The switching frequency of PWM is generally fixed, and the control sampling signal can form a single-loop, dual-loop or multi-loop feedback system to achieve the purpose of voltage regulation, current regulation and constant power.

　　At the same time, some additional functions such as over-current protection, anti-bias magnetization and current equalization can be realized.

　　When repairing a switching power supply, when the rectifier filter circuit and the switch tube are normal, it is usually necessary to check whether the PWM IC and peripheral circuits are normal, which will achieve twice the result with half the effort. PWM ICs basically have IC power supply, reference voltage, drive pulse, current detection and sampling adjustment circuits. The power supply of PWM IC is generally obtained by reducing the voltage of the main power supply through a resistor, which is usually called the startup resistor. If the resistor is open or becomes larger, the power supply to the IC will be low, which will cause the power supply to fail to start.

　　When the power supply is normal, focus on checking whether the reference voltage and drive pulse are normal, then monitor whether the voltage at the current sensing end is normal, and then carefully check the sensing branch. The method to determine the fault of the PWM IC itself is generally to measure the impedance between the pins or input the nominal voltage to the power supply end to observe whether the reference voltage is accurate.

　　Example 1: North American GS anesthesia machine +5V, +12V power board has no output. The primary PWM IC chip of the machine's switching power supply is UC3845, and the fuse and main power components are intact. The plan is to first check the PWM IC chip power supply, reference voltage and current detection pin voltage, and it is found that +300V is normal, and there is no voltage input at pin 7.

　　The reason was that the 100K startup resistor was open. After replacement, the PWM IC was powered normally, the 6-pin output pulse waveform was stable, and the +5V and +12V output voltages were correct.

　　Example 2: Stryker laparoscope monitor, the power supply consists of switch tube BUK456, UC3824 and peripheral circuits. UC3842 was damaged due to short circuit between pin 6 and pin 5, DS pole of BUK456 was broken, current detection resistor was open, and series resistor of pulse output terminal was open. After replacing the above components, it worked normally. If only the damaged components of peripheral circuit were replaced without finding the damage of UC3842 itself, the replaced components would be damaged again at the moment of power on. Therefore, the fault of PWM IC itself should be excluded during maintenance.

　　5 Other power supply components are faulty

　　During maintenance, we often encounter some faults that are not caused by complete damage to electronic components. For example, the capacitance becomes smaller, the hidden contact of the circuit board is poor, the power supply is unstable due to excessive dust or poor heat dissipation, and the power supply stops vibrating due to the failure of some fan control circuits. Since such problems are difficult to detect through traditional methods, we use replacement methods to eliminate them based on experience and analysis.

　　When repairing the power supply of medical equipment, the dust should be handled first. It can be removed with a vacuum cleaner and a high-power air cooler. During the handling process, it is necessary to reduce the static electricity of the human body and prevent the circuit board capacitor from discharging to the human body. For power supplies with a large number of fans, the speed of the fans must be checked, especially those with speed control or speed detection. If you are not sure, you can use the replacement method to solve the problem.

　　Example 1: Hitachi 7170A biochemical analyzer +5V switching power supply, after starting up and working normally for a few minutes, the power indicator light turns from green to off, the +5V output stops, the cooling fan has no obvious abnormality, the power components and PWM IC are normal, but after cleaning, the fan power is not connected and there is no output. After replacing the ordinary 2-wire CPU fan, there is still no output. After the fan speed measurement line is connected and connected to the circuit board, the power input is normal and can work continuously. Therefore, it can be concluded that the original fan has stopped vibrating due to the reduction of speed for a long time. In the past, +24V also had such problems. At that time, the new power module was replaced because the equipment was used urgently and then returned to normal.

　　Example 2: After a long period of continuous operation, the power supply housing of the TOSHIBA 240A Model B became hot and the over-temperature protection function often occurred. This type of failure is generally caused by excessive internal dust or a low internal fan speed, which deteriorates the working environment of the entire power supply. After the power supply was removed and thoroughly dusted, and the fans at the bottom and back of the power supply were replaced, the power supply temperature dropped significantly, the machine worked normally, and there was no failure for 1 year.

　　6 Summary

　　There are many types of medical equipment, and high-power, high-current switching power supplies are widely used in medical equipment. Switching power supply failures account for more than 60% of medical equipment failures. Therefore, mastering the maintenance of switching power supplies is a basic skill for every clinical medical engineer, and it is also a difficult point. This article only discusses the maintenance technology of medical equipment switching power supplies based on actual maintenance experience. It is hoped that more peer experts will provide valuable opinions and conduct further research on medical equipment switching power supplies.